Electroluminescent devices with a barium titanate layer



4, 1954 M. G. CLARKE ETAL 3, 43,682

ELECTROLUMINESCENT DEVICES WITH A BARIUM TITANATE LAYER Filed D80. 20, 1954 lnven t'ors: Nauvice G.Cl,a1-l e, CHT'iL H.WaLl e1-,

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United States Patent "ice 3,143,682 ELECTROLUMINESCENT DEVICES WITH A BARIUM TITANATE LAYER Maurice G. Clarke and Cyril H. Walker, Rugby, England, assignors to The British Thomson-Houston Company Limited, a British company Filed Dec. 20, 1954, Ser. No. 476,222 5 Claims. (Cl. 313-108) This invention relates to devices incorporating electroluminescent material.

Such devices are constructed to provide a support generally of a transparent nature for electrically conductive layers insulated from one another and separated by a layer of electroluminescent material. At least one of the electrically conductive layers is transparent so that the light emitted from the material when energized can be usefully employed.

The construction generally employed involves the use of a sheet of glass as a transparent support, the glass being rendered conductive on one of its faces by means of a transparent conductive film formed thereon. The light emitted by the electroluminescent material then passes out through the transparent conductive film and the glass on which it is formed.

The general object of this invention is to increase the efliciency and performance of electroluminescent devices.

A more specific object is to provide improved means whereby the light emission from the electroluminescent material is enhanced.

In accordance with the invention, the layer of electroluminescent material is separated from an adjacent electrically conductive film by an intermediate layer of material possessing a high dielectric constant, a high dielectric strength, and an ability to serve as a reflector of the light emitted by the electroluminescent material. The present invention thus provides an improvement, as will be more fully described hereafter, over the insulating layer or layers described in co-pending application No. 317,613, filed October 30, 1952, of Shepard Roberts, entitled Electroluminescent Devices, and assigned to the same assignee as the present invention.

The invention may be carried out in a known arrangement in which the electroluminescent material is formed as a layer on a transparent electrically conducting surface of a glass plate which serves both as a support for the electroluminescent material and a medium through which the light emitted thereby can pass. In accordance with the invention, we provide a layer of a material possessing a high dielectric constant, a high dielectric strength, and an ability to serve as a reflector on the layer of luminescent material so that it forms a backing to the layer of luminescent material on the side remote from the glass. Since it is necessary to provide on that face of the layer of electroluminescent material a further electrically conduct'mg film, it may be provided by a coating of electrically conducting paint, or metal deposited in vacuo. The layer of material possessing a high dielectric constant is then intermediate between the layer of luminescent material and the coating of conducting paint.

A layer of material of high dielectric constant can additionally or alternatively be placed next to the transparent electrically conducting surface in order to isolate the phosphor from that surface. A thin layer serves to prevent possible contamination of the phosphor and improve the luminous maintenance of the electroluminescent panel.

The material of high dielectric constant and high reflectance reflects light rays which would otherwise be absorbed in the electrically conducting film, as well as having a low potential gradient across it and enabling a large electrical stress to be placed across the phosphor,

3,143,682 Patented Aug. 4 1964 when an electric potential is applied to the electrically conducting layers. In addition, the presence of the material of high dielectric constant increases the breakdown voltage of the insulation between the electrically conducting films located at both faces of the luminescent material.

If a device designed for a particular voltage is made using half the normal thickness of electroluminescent material and made up to the full thickness by a layer of material of high dielectric constant, it will be found that a greater potential gradient appears across the electroluminescent layer. The loss of light output resulting from the omission of the second half layer of electroluminescent material is compensated by the presence of the high reflectivity material, so that a greater light output, or an equal light output at higher efficiency, is emitted from the device.

To enable the invention to be more readily understood, reference will now be made to the accompanying drawings wherein:

FIG. 1 shows in perspective an electroluminescent panel constructed in accordance with the invention; and

FIG. 2 is a section of a portion of the panel designed to show its internal construction.

Referring to the drawing, we have shown in FIG. 1 a rectangular panel intended to emit light by electroluminescence. The panel is formed of a supporting plate 1 of a light-transmitting material, such as glass, or a transparent polymer or plastic material. The support 1 carries the electroluminescent layer and the conducting films for enabling an operating potential to be applied to the layer whereby it is caused to become luminescent. Terminals are shown at 2 which are connected to the conducting films for applying the operating potential thereto, and the panel is shown as completed by a sealing compound 3 which is arranged to envelop the edges of the support 1 and the layers carried thereby and prevent ingress of the atmosphere between the layers; the sealing compound may consist of an epoxide resin.

As shown more clearly in the section given in FIG. 2, the supporting plate 1 is provided on its lower face with an electrically conducting film 4 on which is positioned a la er 5 of phosphor capable of electroluminescence. The film 4 may consist of a layer of tin oxide formed by spraying tin chloride under conditions of heat. The layer 5 may consist of any of the electroluminescent phosphors known to the art, for instance an electroluminescent zinc sulphide copper-activated phosphor. In one such well-known phosphor, the percentage of copper prior to washing is between 0.05 to 0.1 percent. Other known suitable electroluminescent phosphors are zinc sulphide-zinc oxide activated with copper and manganese, and zinc sulphide-zinc selenide activated with copper.

On the layer 5 of phosphor is provided according to the invention a further layer 6 of a material possessing a high dielectric constant, a high dielectric strength and an ability to serve as a reflector of the light emitted by the electroluminescent material 5 when excited.

Backing the layer 6 is a further electrically conducting film 7. The protective and sealing material 3 extends over the edges of the supporting plate and the films and layers adhering thereto.

Suitable materials for the layer 6 which possess the necessary high dielectric strength, high dielectric constant as well as an ability to serve as a reflector are titanium dioxide and barium titanate, both of which are available in the form of powder which can be applied to the powdered electroluminescent material. The titanium dioxide may be in rutile form. It is preferred to apply the layer 5 of electroluminescent material and the layer 6 of high dielectric constant in the form of suspensions of the powdered materials in a solution of a synthetic insulating resin such as a solution of a methacrylate, for example, polybutyl methacrylate in a solvent such as acetone. When the solvent has evaporated, the particles of powder in the electroluminescent and high dielectric constant materials are maintained in position by the binding efiect of the resin. The resin fills up the interstices between the particles of powder and thus forms coherent uniform layers of dense dispersions of the phosphor and of the high dielectric constant material in insulating resin.

A layer of titanium dioxide may also be applied to either the electrically conducting film or the phosphordielectric layer by the passage of titanium tetra-chloride vapor and water vapor over them at a temperature of 100 to 200 C., whereupon the titanium tetra-chloride hydrolyzes at the surface leaving a deposit of titanium dioxide. It may be necessary to ensure that the surface to be coated is protected from the hydrochloric acid vapor produced during the deposition by a very thin film of additional dielectric materials, such as the methacrylate already mentioned.

The electrically conducting film 7 which is located on the layer of material possessing a high dielectric constant may be formed of a metallic paint, as abovementioned, or of a colloidal suspension of graphite. If a metallic paint is employed, a suitable material for the metal is aluminum, since, if breakdown occurs between the electrically conductive layers under the electric stress, the aluminum particles oxidize and the resulting oxide so formed is a useful insulator which tends to seal the gap dioxide separating the layer of electroluminescent material from an adjacent conductive surface, a brightness increase of at least twice can be obtained over that given by a construction using a layer of electroluminescent material alone between the conducting surfaces. It is also found that an efliciency increase from approximately 3.5 lumens per Watt to approximately 10 lumens per watt is obtained from the improved construction mainly due to the reduction in power factor.

Whereas a preferred embodiment of the invention including specific materials, has been shown and described, it is to be understood that this has been by way of illustrative example and not in order to limit the invention thereto. The scope of the invention is to be determined by the appended claims which are intended to cover any modifications falling within its true spirit and intent.

What we claim as new and desire to secure by Letters Patent of the United States is: 0

1. An electroluminescent device comprising an electrically conductive light-transmitting plate, a layer thereover of electroluminescent phosphor densely dispersed in an insulating resin, a layer thereover of a material of high dielectric constant and high light reflectance and comprising barium titanate densely dispersed in an insulating resin and serving to reflect light from said phosphor layer toward said light-transmitting plate, and a coating thereover of electrically conductive material.

2. An electroluminescent device comprising a pair of electrically conductive layers separated by a first layer of electroluminescent material dispersed and suspended in a dielectric medium and a second layer of barium titanate dispersed and suspended in a dielectric medium, said electrically conductive layers being electrically independent.

3. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of electroluminescent material disposed on said transparent layer, a layer of barium titanate disposed on said layer of electroluminescent material, and a layer of electrically conductive material disposed on said layer of barium titanate, said layers of electrically conductive material being electrically independent.

4. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of electroluminescent material dispersed in a dielectric medium and disposed on said transparent layer, a layer of barium titanate dispersed in a dielectric medium and disposed on said electroluminescent layer, and a layer of electrically conductive material disposed on said layer of barium titanate, said layers of electrically conductive material being electrically independent.

5. An electroluminescent device comprising a pair of electrically independent electrode layers with a layer of electroluminescent material and a layer of barium titanate disposed therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,183,256 Gabler Dec. 12, 19.39 2,436,208 Dressel Feb. 13, 1948, 2,519,722 Turner Aug. 22, 1950 2,566,349 Mager Sept. 4, 1951 2,582,931 Kodama June 13, 1952 2,733,367 Gillson Jan. 31, 1956 2,873,380 Kazan Feb. 10, 19 59 

1. AN ELECTROLUMINESCENT DEVICE COMPRISING AN ELECTRICALLY CONDUCTIVE LIGHT-TRANSMITTING PLATE, A LAYER THEREOVER OF ELECTROLUMINESCENT PHOSPHOR, DENSELY DISPERSED IN AN INSULTING RESIN, A LAYER THEREOVER OF A MATERIAL OF HIGH DIELECTRIC CONSTANT AND HIGH LIGHT REFLECTANCE AND COMPRISING BARIUM TITANTE DENSELY DISSPERSED IN AN INSULATING RESIN AND SERVING TO REFLECT LIGHT FROM SAID PHOSPHOR LAYER TOWARD SAID LIGHT-TRANSMITTING PLATE, AND A COATING THEREOVER OF ELECTRICALLY CONDUCTIVE MATERIAL. 